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Flight dynamics –II Prof. E.G. Tulapurkara Stability and control
Dept. of Aerospace Engg., IIT Madras 1
Chapter 6
Lateral static stability and control - 3
Lecture 21
Topics 6.11 General discussions on control surface
6.11.1 Aerodynamic balancing
6.11.2 Set back hinge or over hang balance
6.11.3 Horn balanace
6.11.4 Internal balance or internal seal
6.11.5 Frise aileron
6.11.6 Tabs – introductory remark
6.11.7 Trim tab
6.11.8 Link balance tab
6.11.9 Servo tab
6.12 Powers boosted and power operated controls and fly-by-wire
6.13 miscelleneous topics
6.13.1 Mass balancing of controls
6.13.2 All movable tail
6.13.3 Elevons
6.13.4 V– tail
6.13.5 Configuration with two vertical tails
6.11 General discussion on controls
In chapters 2 to 5 and in the previous sections of this chapter, some
specific features of elevator, rudder and aileron were considered. In this section
some common features of the controls are dealt with.
6.11.1 Aerodynamic balancing
The ways and means of reducing the magnitudes of Chαt and Chδe are called
aerodynamic balancing.
The methods for aerodynamic balancing are:
Flight dynamics –II Prof. E.G. Tulapurkara Stability and control
Dept. of Aerospace Engg., IIT Madras 2
1. set back hinge,
2. horn balance and
3. internal balance.
6.11.2 Set back hinge or over hang balance
In this case, the hinge line is shifted behind the leading edge of the control
(see upper part of Fig.6.6). As the hinge line shifts, the area of the control surface
ahead of the hinge line increases and from the pressure distribution in Fig.3.3 it
is evident that |Chαt |and |Chδe | would decrease. The over hang is characterized
by cb / cf .Figure 6.6 also shows typical experimental data on variations of Chα
and Chδ with cb / cf. It may be added that the changes in Chα and Chδ also depend
on (a) gap between nose of the control surface and the main surface, (b) nose
shape and (c) trailing edge angle (Fig.6.7a and b).
Fig.6.6 Effect of set back hinge on Chα and Chδ – NACA 0015 Airfoil with blunt
nose and sealed gap (Adapted from Ref.6.1)
Flight dynamics –II Prof. E.G. Tulapurkara Stability and control
Dept. of Aerospace Engg., IIT Madras 3
Fig.6.7a Parameters of control surface - chord lengths
Fig.6.7b Parameters of control surface- shapes of nose and trailing edge
6.11.3 Horn balance
In this method of aerodynamic balancing, a part of the control surface near
the tip, is ahead of the hinge line (Fig.6.8a and b). There are two types of horn
balances – shielded and unshielded (Fig 6.8a). The following parameter is used
to describe the effect of horn balance on Chα and Chδ.
(Areaof horn)×(meanchordof horn)Parameter =
(Areaof control)×(meanchordof control)
Flight dynamics –II Prof. E.G. Tulapurkara Stability and control
Dept. of Aerospace Engg., IIT Madras 4
Figure 6.8b shows the areas of the horn and control surface. Figure 6.8b also
shows the changes ΔChα and ΔChδ due to horn as compared to a control surface
without horn. Horn balance is some times used on horizontal and vertical tails of
low speed airplanes (see Fig.6.8c).
Fig.6.8a Unshielded and shielded horn
Unshielded horn Shielded horn
Flight dynamics –II Prof. E.G. Tulapurkara Stability and control
Dept. of Aerospace Engg., IIT Madras 5
Fig.6.8b Unshielded horn and the changes ΔChα and ΔChδ as compare to control
surface without horn (Adapted from Ref.6.1)
Flight dynamics –II Prof. E.G. Tulapurkara Stability and control
Dept. of Aerospace Engg., IIT Madras 6
Note : The curve representing propeller is circular. Please adjust the resolution of your monitor so that the curve looks circular.
Fig.6.8c Airplane with horn balance on horizontal tail and vertical tail
(Based on drawing of HAMSA-3 supplied by
National Aerospace Laboratories, Bangalore, India)
6.11.4 Internal balance or internal seal
In this case, the portion of the control surface ahead of the hinge line,
projects in the gap between the upper and lower surfaces of the stabilizer. The
upper and lower surfaces of the projected portion are vented to the upper and
lower surface pressures respectively at a chosen chord wise position (upper part
of Fig.6.9). A seal at the leading edge of the projecting portion ensures that the
pressures on the two sides of the projection do not equalize. Figure 6.9 also
shows the changes ΔChα and ΔChδ due to internal seal balance. This method of
Flight dynamics –II Prof. E.G. Tulapurkara Stability and control
Dept. of Aerospace Engg., IIT Madras 7
aerodynamic balancing is complex but is reliable. It is used on large airplanes to
reduce Chα and Chδ.
Fig.6.9 Internal seal and the changes ΔChα and ΔChδ as compared to control
surface with Cb / Cf = 0 (adapted from Ref.6.1)
Remark:
Tab is also used for aerodynamic balancing. See section 6.12.
6.11.5 Frise aileron
The frise aileron is shown in Fig.6.10. The leading edge of the aileron has a
specific shape. The downward deflected aileron has negative Chδ and the upward
deflected aileron has positive Chδ. This reduces the net control force. Further,
Flight dynamics –II Prof. E.G. Tulapurkara Stability and control
Dept. of Aerospace Engg., IIT Madras 8
owing to the special shape of the leading edge, the upward deflected aileron
projects into the flow field and increases the drag. This reduces adverse yaw.
Fig.6.10 Frise aileron
6.11.6 Tabs – introductory remark
The methods of aerodynamic balancing described earlier are sensitive to
fabrication defects and surface curvature. Hence, tabs are used for finer
adjustment to make the hinge moment zero. Tabs are also used for other
purposes. A brief description of different types of tabs is given in the following
subsections.
6.11.7 Trim tab
It is used to trim the stick or bring Ch to zero by tab deflection. After the
desired elevator deflection (δe) is achieved, the tab is deflected in a direction
opposite to that of the elevator so that the hinge moment becomes zero. Since
the tab is located far from the hinge line, a small amount of tab deflection is
adequate to bring Che to zero (Figs.2.16a and b). As the lift due to the tab is in a
direction opposite to that of the elevator, a slight adjustment in elevator deflection
would be needed after application of tab. Though the pilot subsequently does not
have to hold the stick all the time, the initial effort to move the control is not
reduced when this tab is used.
6.11.8 Link balance Tab
In this case the tab is linked to the main control surface. As the main
surface moves up the tab deflects in the opposite direction in a certain proportion
(Fig.6.11). This way the tab reduces the hinge moment and hence it is called
‘Balance tab’.
Flight dynamics –II Prof. E.G. Tulapurkara Stability and control
Dept. of Aerospace Engg., IIT Madras 9
Fig.6.11 Link balance tab
6.11.9 Servo tab
In this case the pilot does not move the main surface which is free to
rotate about the hinge. Instead the pilot moves only the tab as a result of which
the pressure distribution is altered on the main control surface and it attains a
floating angle such that Ch is zero .The action of the tab is like a servo action and
hence it is called “Servo tab”. This type of tab is used on the control surfaces of
large airplanes.
6.12 Power boosted and power operated controls and fly-by-wire
As pointed out earlier (section 3.4.1) the control force increases in
proportion to the cube of the linear dimension of the airplane and to the square of
the flight velocity. Consequently, a low value of Chδ is required, to restrict the
control forces within human limits. It may be as low as 0.0002. This is not
achievable consistently due to sensitive dependence of Chδ on uncertainty in
fabrication. The alternative systems for operation of controls are as follows.
(a) Hydraulic power boosted systems in which the effort of the pilot is boosted by
a hydraulic system. (b) Power operated systems in which the movements of the
Flight dynamics –II Prof. E.G. Tulapurkara Stability and control
Dept. of Aerospace Engg., IIT Madras 10
pilot alter settings of electrical/ electronic systems which in turn cause the
movement of the controls. This led to fly-by-wire system wherein the aircraft
motion (e.g. velocity, angular rates, acceleration, incidence and sideslip) are
sensed by appropriate transducers. Then, optimum response of the airplane is
computed and the control surfaces are actuated to give desired results. This
system requires artificial feel system to give the pilot, an appreciation of the result
of the stick/ pedal movements by him and also to prevent inadvertent excess
movement of control surfaces. This system also needs multi-plexing i.e. alternate
systems to take over if one of the systems fails.
Remark:
(i) Subsection 1.2.2 may be referred to for brief descriptions of the relaxed static
stability and the control configured vehicle (CCV).
(ii) Fly-by light :
In early fly-by wire systems the signals, from flight computer to the control
surface actuators, were sent through wires. Presently, the signals are sent
through fibre optic cables. Hence, the fly-by-wire system is now called “Fly-by-
light” (Ref. 6.2 Chapter 12)
6.13 Miscellaneous topics
6.13.1 Mass balancing of control
This ensures that the c.g. of the control surface lies ahead or on the hinge line.
6.13.2 All movable tail
In some military and large civil airplanes the entire horizontal tail is hinged
and rotated to obtain larger longitudinal control.
6.13.3 Elevons
In a tailless configuration (e.g. concorde airplane) the functions of the
elevator and the aileron are combined in control surfaces called elevons. Like
ailerons they are located near the wing tip but the movable surfaces on the two
wing halves can move in the same direction or in different directions. When they
move in the same direction, they provide pitch control and when they move in
different directions they provide control in roll.
Flight dynamics –II Prof. E.G. Tulapurkara Stability and control
Dept. of Aerospace Engg., IIT Madras 11
6.13.4 V– tail
In some older airplanes the functions of horizontal and vertical tails were
combined in a V-shaped tail. Though the area of the V-tail is less than the sum of
the areas of the horizontal and vertical tail, it leads to undesirable coupling of
lateral and longitudinal motions and is seldom used.
6.13.5 Configuration with two vertical tails
At supersonic speeds the slope of the lift curve (dCL/dα) is proportional to
2 1/21/(M -1) , where M∞ is the free stream Mach number. Thus, CLα and in turn the
tail effectiveness decreases significantly at high Mach numbers. Hence some
military airplanes have two moderate sized vertical tails instead of one large tail.
For example see MiG-29M (Fig.6.12). Reference 1.13, chapter 5 mentions that
for this configuration the quantity dσ
η (1+ )dβv
, mention in Eq.(5.19) and (5.20),
has a higher value as compared to the configuration with single vertical tail
located in the plane of symmetry.
Flight dynamics –II Prof. E.G. Tulapurkara Stability and control
Dept. of Aerospace Engg., IIT Madras 12
Fig.6.12 Airplane with two vertical tails MIG-29M
(Adapted from: http://www.defenseindustrydaily.com)